1. Field of the Invention
The present invention relates to a multi-pipe heat exchanger apparatus and a method of producing the heat exchanger apparatus. The heat exchanger apparatus includes a body unit, and a plurality of heat conductive pipes provided in the body unit. First passages for a first fluid are formed in the heat conductive pipes, and second passages for a second fluid are formed between the heat conductive pipes.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or the air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
In the fuel cell, a heat exchanger is used for heating the air supplied to the cathode by heat exchange with the exhaust gas (reactant gas after consumption) or the like. For example, as one type of the heat exchanger, a multi-pipe heat exchanger as disclosed in Japanese Laid-Open Patent Publication No. 8-261679 is known.
As shown in FIG. 14, the multi-pipe heat exchanger includes a plurality of heat conductive pipes 1 arranged in parallel. Opposite ends of the heat conductive pipes 1 are joined to tubular plates 2a, 2b. The heat conductive pipes 1 are supported by the tubular plates 2a, 2b. Each of the heat conductive pipes 1 has a substantially oval cross section. At least one rib (not shown) extends in the heat conductive pipe 1 in parallel with the longitudinal direction.
In the conventional technique, in order to increase the efficiency in heat exchange between the fluid flowing inside the heat conductive pipes 1 and the fluid flowing outside the heat conductive pipes 1 or in order to increase the heat conductive surface area, it is necessary to increase the number of the heat conductive pipes 1. However, it is necessary to decrease the diameter of the heat conductive pipes 1 for increasing the number of the heat conductive pipes 1. Therefore, the pressure loss of the fluid flowing inside the heat conductive pipes 1 becomes large. Thus, it is not possible to increase the amount of fluid supplied into the heat conductive pipes 1, and it is not possible to increase the efficiency in the heat exchanger apparatus.
Further, due to the space constraint, there is a limit in the number of the heat conductive pipes 1 which can be increased. The volume ratio of the fluid flowing inside the heat conductive pipes 1 to the fluid flowing outside the heat conductive pipes 1 is not constant. Therefore, it is difficult to improve the heat exchange efficiency.
Further, as the number of the heat conductive pipes 1 increases, the surface area of the thick portion of the heat conductive pipes 1 increases. Thus, due to the excessive thickness of the heat conductive pipes 1, the heat capacity is increased, and the thermal efficiency is lowered significantly.